First of all you need this script: https://www.gta5-mods.com/scripts/pickups

Put the 'pickups' folder in your 'scripts' folder (will replace InfamouSabre's 'pickups' folder)

Once installed go to vanilla weapons.meta (GTAV V\update\update.rpf\common\data\ai) and search for the following:

<Name>AMMO_BIRD_CRAP</Name>

If you get a search error try finding it manually, it's line 1959.
Once you do that you will be able to see the whole thing:

<Item type="CAmmoThrownInfo">
          <Name>AMMO_BIRD_CRAP</Name>
          <Model>w_ex_birdshat</Model>
          <Audio />
          <Slot />
          <AmmoMax value="25" />
          <AmmoMax50 value="25" />
          <AmmoMax100 value="25" />
          <AmmoMaxMP value="25" />
          <AmmoMax50MP value="25" />
          <AmmoMax100MP value="25" />
          <AmmoFlags />
          <Damage value="0.000000" />
          <LifeTime value="4.000000" />
          <FromVehicleLifeTime value="5.000000" />
          <LifeTimeAfterImpact value="0.000000" />
          <LifeTimeAfterExplosion value="0.000000" />
          <ExplosionTime value="0.000000" />
          <LaunchSpeed value="1.000000" />
          <SeparationTime value="0.000000" />
          <TimeToReachTarget value="5.000000" />
          <Damping value="0.050000" />
          <GravityFactor value="1.000000" />
          <RicochetTolerance value="0.000000" />
          <PedRicochetTolerance value="0.000000" />
          <VehicleRicochetTolerance value="0.000000" />
          <Explosion>
            <Default>EXP_TAG_BIRD_CRAP</Default>
            <HitCar>DONTCARE</HitCar>
            <HitTruck>DONTCARE</HitTruck>
            <HitBike>DONTCARE</HitBike>
            <HitBoat>DONTCARE</HitBoat>
            <HitPlane>DONTCARE</HitPlane>
          </Explosion>
          <FuseFx />
          <TrailFx />
          <TrailFxUnderWater />
          <TrailFxFadeInTime value="0.000000" />
          <TrailFxFadeOutTime value="0.000000" />
          <PrimedFx />
          <DisturbFxDefault />
          <DisturbFxSand />
          <DisturbFxWater />
          <DisturbFxDirt />
          <DisturbFxFoliage />
          <DisturbFxProbeDist value="0.000000" />
          <DisturbFxScale value="0.000000" />
          <LightOnlyActiveWhenStuck value="false" />
          <LightFlickers value="false" />
          <LightBone />
          <LightColour x="0.000000" y="0.000000" z="0.000000" />
          <LightIntensity value="0.000000" />
          <LightRange value="0.000000" />
          <LightFalloffExp value="0.000000" />
          <LightFrequency value="0.000000" />
          <LightPower value="0.000000" />
          <CoronaSize value="0.000000" />
          <CoronaIntensity value="0.000000" />
          <CoronaZBias value="0.000000" />
          <ProjectileFlags>DestroyOnImpact ProcessImpacts DontFireAnyEvents</ProjectileFlags>
          <ThrownForce value="0.000000" />
          <ThrownForceFromVehicle value="5.000000" />
        </Item>
      </Infos>
    </Item>

Underneath this^^^ add the following lines:

  <Item type="CAmmoInfo">
          <Name>AMMO_79257</Name>
          <Model />
          <Audio />
          <Slot />
          <AmmoMax value="150" />
          <AmmoMax50 value="150" />
          <AmmoMax100 value="150" />
          <AmmoMaxMP value="250" />
          <AmmoMax50MP value="1000" />
          <AmmoMax100MP value="9999" />
          <AmmoFlags />
        </Item>

Once you do that go to your add-on MG-42 dlc folder (GTA V\update\x64\dlcpacks\metroidguys-addon-guns\dlc.rpf\common\data\ai)
extract the 'addon-weapons.meta' file and wherever you extracted it you can open it with notepad or notepad++

Then in the weapon meta for your add-on weapon, where you see "<AmmoInfo ref="AMMO_RIFLE" />" (search it)

change it to "<AmmoInfo ref="AMMO_79257" />"

That's it and enjoy your icon (hey this is the least tedious way of getting an icon, trust me).

